Method of producing p-n junctions in



F/GZ

S. E. MAYER METHOD OF PRODUCING P-N JUNCTIONS IN SEMI-CONDUCTORMATERIALS Filed May 25, 1955 F/GS.

Inventor S. E. MAYER A ttorney United States Pat METHOD OF PRODUCING P-NJUNCTIONS IN SEMI-CONDUCTOR MATERIALS Simon Ernst Mayer, London,England, assignor to International Standard Electric Corporation, NewYork, N. Y., a corporation of Delaware The present invention relates tothe treatment and preparation of electric semi-conducting materials suchas are used for electric rectifiersor crystal triodes, or energydetectors. I

Hitherto, single crystals of certain elements such as silicon orgermanium taken from group 4 of the periodic table of elements have beencommonly used as semiconductor materials, but other semiconductorsconsisting of chemical compounds between certain elements from groups 3and 5, such as aluminium antimonide, are now being investigated.

The electrical conductivity of semi-conductors such as germanium isgenerally considered to depend on the presence in the crystal of acertain number of lattice irregularities.

These can be two kinds:

(1) The kind in which a small percentage of the atoms of the crystal arereplaced by some other element or elements; and

(2) The kind in which the crystal has lattice defects, that is, apercentage of the atoms are missing altogether from the lattice, or aredisplaced from their proper positions.

Lattice irregularities of type 1 may be produced by the addition to thebasic semiconductor material (such as germanium or silicon) of a smallpercentage of a donor element, such as arsenic, from group 5 of theperiodic table. In that case there are produced a small number of easilydetached electrons which can act as current carriers, and thesemi-conductor is then said to be of the N-type, or to haveN-type'conductivity. Type 1 irregularities may also be produced by theaddition of a small percentage of an acceptor element, such as aluminum,from group 3 of the periodic table. In that case the crystal lattice isdeficient in electrons and there are present a number of sites in thelattice where an electron is missing, which are called holes, and whichact like positive current carriers. The semiconductor is then said to beof the P-type, or to have P-type conductivity. It should be pointed out,also, that gold has been found to be a very effective acceptor element.

Type 2 lattice irregularities, namely lattice defects,

have substantially the same effect as the addition of acceptor elementsand produce P-type conductivity.

For convenience, the elements added, or present in small percentages,for controllingthe conductivity type of the semiconductor, will becalled significant impurities, and will be referred to as donor oracceptor impurities according as the corresponding elements are donor oracceptor elements.

When lattice irregularities due to any or all of the above describedcauses are present, the resulting type of conductivity of thesemiconductor is determined by the preponderating cause. For example, ifboth donor and acceptor impurities are present in a region of thesemiconductor, the conductivity of that region will be Ice N-type orP-type according as the donor or the acceptor r impurity atoms are inthe majority.

In the manufacture of semi-conductor devices, it is often desired toproduce a semi-conducting crystal with two or more regions ofalternately P- and N-type con ductivity separated from one another bywhat are called P-N junctions. One method of doing this is describedinthe specification of British Patent No. 753,133. Very briefly, itconsists in starting with a slice from a semiconducting crystal havingN-type conductivity, depositing on one surface of the slice a thin filmof an acceptor impurity, and then heating the crystal to a suitabletemperature for difiusing the acceptor impurity into the crystal. Ifsufiicient quantity of the acceptor impurity is used, a layer of thecrystal will be converted to P-type conductivity, and a P-N junctionwill be formed at a depth below the surface depending on the heattreatment.

The difficulty which has-been encountered with this method is that theheat treatment causes the significant impurity to spread very rapidlyover all the surfaces of the crystal slice, so that a P-type layer isformed on every such surface, and has to be ground off where it is notwanted;

The object of the present invention is to prevent the unwanted spreadingof the deposited significant impurity over the surface of the crystal.It is believed that this spreading is largely due to the existence atthe surface of relatively large numbers of lattice defects of the kindin which atoms are missing from the lattice.

Accordingly, the invention provides a remedy which has been found to beeffective in preventing this un wanted spreading. This remedy consistsin depositing on the surface or surfaces of the semiconductor, which isor are not to be covered, by the significant impurity, a film of aninhibiting element which is isoinorphous with the basic semiconductormaterial but different therefrom (or a compound of such an element), atsome stage before the heat treatment for diffusing the significantimpurity into the semiconductor is applied. Stated more specifically,the inhibiting element is: taken from group 4 of the periodic table inthe case where the semiconducting crystal is silicon or germanium.

The invention also provides a semiconductor device employing asemiconductor which has been treated in the manner just stated.

In practice, the most convenient inhibiting element for a germaniumcrystalhas been found to be silicon, either in the elementary form, orin the form of silicon monoxide.

One example of the process according to the invention will be explainedwith reference to the drawing accompanying the provisionalspecification, in which Figs. 1 to 6 show sectional views of asemiconducting crystal illustrating stages in the process.

In these figures the thickness of layers and films on the surface of thecrystal is greatly exaggerated in order to show the details clearly.

Referring to Fig. 1, a slice of a germanium crystal I, assumed to be ofthe N-type, and having a specific resistanceof about 5 ohm-cm, forexample, is shown in section. The crystal slice may be circular in plan,or

surfaces of the slice, a thin film or coating of silicon 3 monoxide 3.This film may for example be 10- inch thick.

A film 4 (Fig. 2) of an acceptor impurity element, such as gold, forexample, is then evaporated or otherwise deposited on the upper face ofthe crystal. This layer may be about 2 l inch thick, for example, andthe crystal is then heated at a temperature of about 850 C. for about 4hours in order to diffuse the gold into the crystal.

The atoms of gold will convert the upper portion of the crystal toP-type conductivity, thereby producing a P- type region or layer about0.008 inch thick, shown at 5 in Fig. 3. Between the layer 5 and themainportion 6 of the crystal there will be produced a P- N junction 7, shownas a dotted line, about 0.008 inch below the surface. As alreadyexplained, the film 3 prevents the gold from spreading over the wholesurface of the crystal during the heat treatment.

If a second PN junction is required, the process may be repeated. Asshown in Fig. 4, the crystal with its P- and N-type regions 5 and 6already formed has again deposited on its upper surface, by evaporationor otherwise, a thin film 8 of a suitable donor impurity element such asantimony. This film may for example, be about inch thick. The crystal isagain heated at a temperature of about 850 C. for about five minutes inorder to diffuse the donor element into the P-type layer, therebyreconverting its upper portion to N-type conductivity. In this way anN-type layer 9 (Fig. 5) is formed, which may be 0.001 inch thick forexample. A second P-N junction 10 is thus formed, the crystal beingdivided into three regions with respectively N-, P-, and N-typeproperties. As before the spread of the donor impurity element over thewhole surface of the crystal is prevented by the film 3.

It will be evident from Figs. 1, 2 and 3 that if the portion of the film3 on the lower surface of the crystal 1 be omitted (the portions on thesides of the crystal still remaining, a gold film may be deposited onboth the upper and lower surfaces. After diffusion for 4 hours at 850C., a crystal as shown in Fig. 6, with three regions 11, 12, 13 havingrespectively P-, N-, and P-type conductivity will be produced, separatedby P-N junctions 14, 15 about 0.008 inch from the upper and lowersurfaces.

It will be evident also that a crystal with any number of regions withalternately P- and N-type conductivity may be produced by repeating theprocess described with reference to Figs. 1 to 5 using deposited filmsof alternately acceptor and donor elements on the upper surface of thecrystal.

It should be mentioned that the heating necessary for the significantimpurities generally produces lattice defects in sufficient number toconvert the whole crystal to P-type conductivity irrespective of thepresence of donor impurities. Therefore, in order to remove theseunwanted lattice defects, the crystal should be annealed at atemperature of about 500 C. after heating for the diffusion of thesignificant impurities.

It will be understood that although in the above process gold is used asan acceptor element, various others, such as aluminium, gallium, orindium could be used. Likewise, for a donor element, phosphorus, arsenicor hismuth, for example, could be used instead of antimony.

While the preferred material for the film 3 is silicon monoxide, puresilicon could alternatively be used, or some other element from group 4of the periodic table, which however, should be different from thematerial of which the crystal 1 consists.

It should be understood also that it is not essential that the basicsemiconductor material should be germanium; silicon could be used, orone of the compound materials mentioned above. Further, the originalcrystal could be P-type instead of N-type, in which case a donorimpurity element would be used for the film 4 in Fig. 2 instead of anacceptor element, and an acceptor element for the film 8 in Fig. 4instead of a donor element; and the crystal produced would then be ofthe P-N-P type, like that shown in Fig. 6.

The thickness of the deposited films, and the time and temperature usedfor the diffusion process, which have been given above as examples, arenot in any sense essential to the invention, and will be variedaccording to the conditions which have to be met.

It should be added that the inhibiting film 3 can be applied afterdepositing the film of the significant impurity on the surface of thecrystal instead of before,

but it must be present before any heating for diffusion of the impurityinto the semiconductor. When several successive diffusion processes areapplied, it will not gen- 7 erally be necessary to reapply the film 3after each diffusion process.

It will be understood that a semiconductor having one or more P-Njunctions produced in the manner explained above may be provided in anysuitable way with electrodes to form a rectifier or crystal triode, orother serniconductor device.

While the principles of the invention have been described above inconnection with specific embodiments, and particular modificationsthereof, it is to be clearly understood that this description is madeonly by way of example and not as a limitation on the scope of theinvention.

What I claim is:

1. A method of treating a body of semiconductor crystalline materialselected from the class consisting of germanium and silicon and of givenconductivity type for the purpose of producing therein a P-N junction,comprising the steps of depositing on a given surface of the body a filmof a significant impurity element of the kind adapted, when added to thesemi-conductor material, to produce conductivity of the type opposite tothe given type, and heating the body to a temperature sufficient tocause the significant impurity element to be diffused into the body,characterised in this, that at some stage before the body is heated,there is deposited on all other surfaces of the body except the givensurface a film of a material selected from the class consisting of anele ment and an inorganic compound of such element taken from group IVof the periodic table which is isomorphous with the semi-conductormaterial, but different therefrom.

2. A method of treating a germanium crystal of given conductivity typefor the purpose of producing a P-N junction below a given surface of thecrystal, which comprises the steps of depositing on all surfaces of thecrystal other than the given surface a film of a material selected fromthe class consisting of silicon and an inorganic compound of silicon,then depositing on the given surface a film of a first significantimpurity element of the kind adapted, when added to germanium, toproduce conductivity' of the type opposite to the given type, and thenheating the crystal to a temperature sufiicient to cause the impurityelement to be diffused into the crystal.

3. A method according to claim 2 which comprises the additional steps ofdepositing on the given surface a film of a second significant impurityelement of the kind adapted, when added to germanium, to produceconductivity of the given type, and then heating the crystal to atemperature sufiicient to cause the impurity element to be diffused intothe crystal, for the purpose of producing a second P-N junction belowthe given surface.

4. A method of treating a germanium crystal slice of given conductivitytype for the purpose of producing two P-N junctions between the twoopposite faces of the slice, which comprises the steps of depositing onthose surfaces of the crystal other than the said faces a film materialsc-,

lected from the class consisting of silicon, and an in organic compoundof silicon, then depositing on each of crystal is of N-typeconductivity, and in which the said first the said faces a film of afirst significant impurity element significant impurity element is gold,and in which the said of the kind adapted, when added to germanium, toprosecond impurity element is antimony.

duce conductivity of the type opposite to the given type, ReferencesCited in the file of this patent and then heating the crystal toatemperature sufliclent to 5 cause the said first significant impurityelement to be dif- UNITED STATES PATENTS fused into the crystal slicefrom each of the said faces. 2,473,887 Jennings June 21, 1949 5. Amethod according to claim 4- in which the said 2,560,594 Pearson July17, 1951 inorganic compound of silicon is silicon monoxide. 2,597,028Pfann May 20, 1952 6. A method according to claim 4 in which the said 102,701,326 Pfann et al. Feb. I, 1955

1. A METHOD OF TREATING A BODY OF SEMI-CONDUCTOR CRYSTALLINE MATERIALSELECTED FROM THE CLASS CONSISTING OF GERMANIUM AND SILICON AND OF GIVENCONDUCTIVITY TYPE FOR THE PURPOSE OF PRODUCING THEREIN A P-N JUNCTION,COMPRISING THE STEPS OF DEPOSITING ON A GIVEN SURFACE OF THE BODY A FILMOF A SIGNIFICANT IMPURITY ELEMENT OF THE KIND ADAPTED, WHEN ADDED TO THESEMI-CONDUCTOR MATERIAL, TO PRODUCE CONDUCTIVITY OF THE TYPE OPPOSITE TOTHE GIVEN TYPE, AND HEATING THE BODY TO A TEMPERATURE SUFFICIENT TOCAUSE THE SIGNIFICANT IMPURITY ELEMENT TO BE DIFFUSED INTO THE BODY,CHARACTERISED IN THIS, THAT AT SOME STAGE BEFORE THE BODY IS HEATED,THERE IS DEPOSITED ON ALL OTHER SURFACES OF THE BODY EXCEPT THE GIVENSURFACE A FILM OF A MATERIAL SELECTED FROM THE CLASS CONSISTING OF ANELEMENT AND AN INORGANIC COMPOUND OF SUCH ELEMENT TAKEN FROM GROUP IV OFTHE PERIODIC TABLE WHICH IS ISOMORPHOUS WITH THE SEMI-CONDUCTORMATERIAL, BUT DIFFERENT THEREFROM.